[3.10] bpo-25130: Add calls of gc.collect() in tests to support PyPy (GH-28005) (GH-28027)

(cherry picked from commit 2a8127cafe)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/_test_multiprocessing.py

@@ -611,6 +611,7 @@ def test_lose_target_ref(self):
         del c
         p.start()
         p.join()
+        gc.collect()  # For PyPy or other GCs.
         self.assertIs(wr(), None)
         self.assertEqual(q.get(), 5)
         close_queue(q)
@@ -2667,6 +2668,7 @@ def test_release_task_refs(self):
         self.pool.map(identity, objs)
 
         del objs
+        gc.collect()  # For PyPy or other GCs.
         time.sleep(DELTA)  # let threaded cleanup code run
         self.assertEqual(set(wr() for wr in refs), {None})
         # With a process pool, copies of the objects are returned, check
@@ -4197,6 +4199,7 @@ def setUp(self):
         util._finalizer_registry.clear()
 
     def tearDown(self):
+        gc.collect()  # For PyPy or other GCs.
         self.assertFalse(util._finalizer_registry)
         util._finalizer_registry.update(self.registry_backup)
 
@@ -4208,12 +4211,14 @@ class Foo(object):
         a = Foo()
         util.Finalize(a, conn.send, args=('a',))
         del a           # triggers callback for a
+        gc.collect()  # For PyPy or other GCs.
 
         b = Foo()
         close_b = util.Finalize(b, conn.send, args=('b',))
         close_b()       # triggers callback for b
         close_b()       # does nothing because callback has already been called
         del b           # does nothing because callback has already been called
+        gc.collect()  # For PyPy or other GCs.
 
         c = Foo()
         util.Finalize(c, conn.send, args=('c',))